1. Field of the Invention
The present invention relates to a lamp for selectively and externally emitting light of a desired wavelength range using a light interference film.
2. Description of the Prior Art
In a recently proposed halogen lamp, an infrared ray reflecting film through which visible light passes is formed on the surface of the tubular bulb. Of the light emitted by the filament, infrared light is reflected from the reflecting film and returned to the filament. Thus, the returning infrared light heats the filament and the emitting efficacy is improved. At the same time, the amount of infrared light emitted outside the lamp is reduced.
Such an infrared ray reflecting film is formed with layers of a low-refractive index layer of silicon oxide (SiO.sub.2) or the like and a high-refractive index layer of titanium oxide (TiO.sub.2) or the like. The film can selectively transmit or reflect light of desired wavelength utilizing light interference, particularly by controlling the thickness of each layer. This type of film is called a light interference film.
In a conventional lamp of this type, during operation over a long period of time, the light interference film may cause cracking or peeling. This phenomenon is particularly notable in halogen lamps having a high operation temperature and incandescent lamps operated by repeating short lighting intervals.
In view of this problem, Japanese Patent Disclosure (Kokai) No. 57-124301 discloses a film formed by alternately stacking a low-refractive index layer of silicon oxide (silica) and a high-refractive index layer of aluminua (Al.sub.2 O.sub.3), zirconium oxide (ZrO.sub.2) and/or titanium oxide. Tin and/or zirconium is added to the silica low-refractive index layer.
When a light interference film of the type described in the above-mentioned Disclosure is applied to a halogen lamp having a bulb consisting of a hard glass such as quartz glass or borosilicate glass, cracking or peeling of the light interference film is observed upon frequent on/off operations or operation over a long period of time. A satisfactory performance cannot be obtained when this type of light interference film is used in such a lamp.
Japanese Patent Disclosure (Kokai) No. 57-161809 discloses a TiO.sub.2 /SiO.sub.2 /TiO.sub.2 three-layered film for use in a reflector, a decorative color glass, a mirror or a filter. This Disclosure also discloses the use of phosphorus pentoxide in an amount of 0.5 to 3% by weight based on the weight of SiO.sub.2. However, when this three-layered film is used in a lamp of the type described above, a satisfactory light interfering effect cannot be obtained. That is, the reflectance of infrared light is low. Further, this three-layered film cannot solve the problems mentioned above.